The development and use of solid polymer electrolytes has increased the efficiency and reduced the size of electrochemical cells. U.S. Pat. No. 4,416,747 (Menth et al.) discloses an individual electrolysis cell bounded by bipolar plates and having a solid electrolyte made of a polymer of perfluorinated sulfonic acid (such as that available under the trademark NAFION from E.I. duPont Company, Wilmington, Del.) with a surface coating centrally located between current-collectors and adjoining open metallic structures. A plurality of these individual cells may be integrated together between end plates so that the cells are electrically connected in series, hydrodynamically connected in parallel, and combined to form a block. However, these membranes must be kept moist at all times, since it acts as a conductor only when it is wet.
U.S. Pat. No. 4,417,969 (Ezzell et al.) discloses ion exchange membranes having sulfonic acid groups. The membrane is a polymer having a substantially fluorinated backbone and recurring pendant sulfonic acid groups represented by the following general formula: EQU --O(CFR.sub.1)b(CFR.sub.2)aSO.sub.3 Y
where a and b are independent integers from zero to three with the condition that the sum of a and b must be at least one; R1 and R2 are independently selected from the group consisting of a halogen and a substantially fluorinated alkyl group having one or more carbon atoms; and Y is hydrogen or an alkali metal.
NAFION is a sulfonic acid membrane having a substantially fluorinated backbone and pendant groups according to the following structure: EQU --OCF.sub.2 CF(CF.sub.3)--0--CF.sub.2 CF.sub.2 SO.sub.3 H
Both NAFION 117 (all NAFION products are available from E.I. duPont Company, Wilmington, Del.) and NAFION 115 have equivalent weights of 1100 with thicknesses of 7 mils (175 .mu.m) and 5 mils (125 .mu.m), respectively.
The need for water to support proton conduction in membranes has been addressed in a number of ways. In fuel cells it would initially appear that since water is the product, sufficient water would be already present. Unfortunately, the water formed in a fuel cell is inadequate to maintain membrane hydration except under special conditions. Each proton that moves through the membrane drags at least two or three water molecules with it. As the current density increases the number of water molecules moved through the membrane also increases. Eventually the flux of water being pulled through the membrane by the proton flux exceeds the rate at which water is replenished by diffusion. At this point the membrane begins to dry out, and its internal resistance increases. This sets a relatively low limit on the current density that can be maintained by back diffusion from the cathode surface.
This problem has typically been addressed by adding water, as vapor, to the hydrogen containing stream, or to both gas streams (fuel and oxidizer). There is no doubt that this method works, and high power densities can be achieved. Unfortunately, humidifiers presently available are quite large, heavy and complex.
The simplest way to humidify a gas stream is to pass the gas as a stream of fine bubbles through water. As long as the gas has sufficient contact time with the water, controlling the temperature of the water controls the amount of water in the gas stream. This method works quite well at low gas flow rates, but problems begin to arise as the required gas flow rate increases. To fully saturate a gas with water requires either small bubbles, ideally under 0.5 mm in diameter, or a tall column to allow enough contact time to insure complete saturation. Operating a humidifier under conditions where a gas does not have sufficient contact time to become fully saturated results in the gas carrying a varying amount of water. This unstable operation is undesirable and unacceptable in certain applications, such as the humidification of reactant gases fed to electrochemical devices.
When bubble column type humidifiers are used to saturate a gas stream, they can become rather large. For example, if a contact time of 0.5 seconds is required to saturate the bubbles with water, the column will need to be at least 19 cm tall (based on Stokes law velocity of 38.2 cm/sec for a 0.5 mm bubble of air in water at 80.degree. C.). For a flow rate of one liter of gas per minute, as 0.5 mm bubbles with an average spacing of 0.5 mm, a water volume of over 300 cm.sup.3 is required, with a similar or greater volume for the reverse portion of the convective flow produced by the gas lifting the water. Additional volume is required for the disperser to form the bubbles and for a reserve of water to replenish that lost to evaporation. The resulting humidifier has a volume of over one liter, and any increase in gas flow rate will require an even larger volume.
U.S. Pat. No. 4,973,530 issued to Vanderborgh et al. entitled FUEL CELL WATER TRANSPORT discloses that the moisture content and temperature of a gas can be regulated throughout traverse of the gas in a fuel cell incorporating a solid polymer membrane. Each cell has a flow field incorporating a membrane for effective water transport to a gas as it passes to a second flow field where chemical reactions occur. In this mainer, the temperature and humidity of a gas is effected at each cell. Humidification is achieved by passing the fuel cell gas over the face of a membrane section that is in contact with liquid water on the opposite face.
Another method has been used to humidify a gas inside a proton exchange membrane fuel cell stack assembly, or stack itself. This is accomplished utilizing an internal "dummy cell" dedicated solely to gas humidification. U.S. Pat. No. 5,200,278 issued to Watkins et al. entitled INTEGRATED FUEL CELL POWER GENERATION SYSTEM discloses a fuel cell stack having a humidification section and an electrochemically active section, wherein the humidification section imparts water vapor to an inlet hydrogen containing fuel stream and an inlet oxygen containing oxidant stream. This is done with a membrane humidifier, in which a stream of water is located on one side of a planar membrane sheet of a water permeable material and the gas stream flows on the other side. This method uses the heat of the cell itself to evaporate the water. This method is advantageous because it eliminates the need for an external source of heat to humidify the gas streams. However, the method is also disadvantageous, first, because it limits the humidification of the gas streams to a dew point that is essentially the same as the fuel cell operating temperature and, second, because it also adds to the size of the fuel cell stack. Since the humidifier is a structural part of the stack, it has to be built to serve as a supporting member. This can increase the weight and size of the system by a greater amount than is required for an external humidification system. Extra weight is always a disadvantage.
U.S. Pat. No. 5,382,478 issued to Chow et al. entitled ELECTROCHEMICAL FUEL CELL STACK WITH HUMIDIFICATION SECTION LOCATED UPSTREAM FROM THE ELECTROCHEMICALLY ACTIVE SECTION similarly discloses a fuel cell with a "dummy cell" type humidification section, but teaches that the inlet fuel and oxidant streams should be introduced into the humidification section without first being directed through the electrochemically active section. In this manner, Chow reduced the number of manifold openings in the active section and increases the area available for electrochemical reactions. However, this humidifier suffers the same disadvantages as the '278 patent described above.
Still another humidification method is to inject water directly into either the manifold of a cell or stack, or a gas line leading into the manifold. The water is injected in such a manner as to form a mist in the gas line. As the gas stream is heated by the fuel cell, the water, which has a high surface area due to its small droplet size, quickly evaporates. This type of humidifier produces a very compact humidification system. The amount of water in the gas stream can easily be controlled by metering the liquid water feed into the fuel cell. While this is a good system for fuel cell stacks in the kilowatt range and larger, it is not an effective or efficient system for smaller systems. The disadvantage of direct injection is the difficulty encountered in forming a steady consistent mist at low water flow rates. For instance, a nominally 1 kW proton exchange membrane fuel cell stack consisting of six cells, each at 0.6 V, operating at 85.degree. C. with both the fuel and air streams humidified, requires about 10.3 grams of water per minute to humidify its air stream, assuming a 2:1 air to current stoichiometry at 30 psig. This amount is easily meterable on a consistent basis. A smaller stack, generating 300 W at 70.degree. C. requires only 1.50 grams of water per minute under the same feed conditions. This amount can be metered, but the higher precision required to maintain a smooth flow at the lower feed rate results in the smaller stack actually requiring a more complex humidifier. In the case of an even smaller stack operating at 30W, and the same operating conditions as above, the feed rate drops to 0.150 grams of water per minute for the air stream, and even less for the fuel gas stream. At these feed rates, maintaining a steady flow is extremely difficult. Using a mist type humidifier under these conditions makes controlling the humidifier the most difficult part of operating the stack.
Thus, there remains a need for a humidifier that can deliver a precise and consistent amount of water at a flow rate as low as about 0.1 grams per minute to a gas stream. It would be desirable if the humidifier were small, lightweight and easy to control.